Acid-Base Equilibrium; Infant Formula/chemistry; Infant Formula/metabolism; Milk, Human/chemistry; Milk, Human/metabolism; Minerals/administration & dosage; Body Weight/physiology; Calcium, Dietary/administration & dosage; Calcium, Dietary/blood; Calcium, Dietary/urine; Chlorides/administration & dosage; Chlorides/blood; Chlorides/urine; Diet/methods; Dietary Supplements; Energy Intake/physiology; Food, Fortified; Humans; Infant Nutritional Physiological Phenomena; Infant, Low Birth Weight; Infant, Newborn; Infant, Premature; Kidney/metabolism; Magnesium/administration & dosage; Magnesium/blood; Magnesium/urine; Milk Proteins/administration & dosage; Milk Proteins/blood; Milk Proteins/urine; Minerals/blood; Minerals/urine; Phosphorus, Dietary/administration & dosage; Phosphorus, Dietary/blood; Phosphorus, Dietary/urine; Potassium, Dietary/administration & dosage; Potassium, Dietary/blood; Potassium, Dietary/urine; Sodium, Dietary/administration & dosage; Sodium, Dietary/blood; Sodium, Dietary/urine